Golf club head preferably has: low stiffness in view of attaining high restitution; high fatigue strength in view of durability, and small density of the material thereof in view of reducing weight. Responding to these requirements, titanium alloy-made golf clubs are widely used in recent years, (refer to Patent Document 1).
The stiffness of the club head expresses the restitution force at impact of ball. Accordingly, lower stiffness attains longer driving distance owing to what is called the “spring-like effect”. Since the stiffness of the face is proportional to cube of the face thickness, thinner face is preferable.
Since the face has to have a certain level of fatigue strength to endure the deflection of the face at impact of ball, higher fatigue strength is preferable. With a material having high fatigue strength, the club head allows longer driving distance without face-damage caused by ball.
From the point of maneuverability of a golf club, lower density of the material for the face is preferable. When the weight of the face portion is large, the center of gravity of the club head moves toward the face, which narrows the area of what is called the “sweet spot”.
In the above-described circumstance, the golf clubs which allow longer driving distance than ever have been widely distributed in recent years. As a result, the golf game which should be a competition of skill of players significantly depends on the superiority of tools. The tendency might loose the attractiveness of the golf game as a competition. Responding to the movement, there has been decided to regulate the coefficient of restitution (COR) of the club head to 0.83 or below from 2008, (the restriction has already been enforced for the tournaments of pro-golfers).
If that small coefficient of restitution is to be satisfied by existing materials, however, the face has to become thicker, which increases the club head weight and moves the center of gravity of the club head toward the face, thereby raising a problem of maneuverability.
With the above-described background, a golf club head which is further easy-to-hit while suppressing the increase in the coefficient of restitution is wanted. That type of club head is, however, not developed.
Patent Document 1: Japanese Patent. Laid-Open No. 2003-38690